Design Patterns with PlantUML: Composite Pattern

This post gives a brief overview about the Composite Pattern. The post is part of a series about software design patterns and their UML representations with the help of PlantUML.

The article aims at providing a very short description of the general idea of the pattern in the first part. This also involves a descriptive UML diagram. Then, the second part provides the PlantUML code for the diagram so that these posts can also be used as a source of design patterns in PlantUML syntax.

What is the Composite Pattern?

According to Wikipedia, the Composite pattern describes a group of objects that is treated the same way as a single instance of the same type of object. The intent of a composite is to “compose” objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly.

What problems can the Composite design pattern solve?

A part-whole hierarchy should be represented so that clients can treat part and whole objects uniformly.

A part-whole hierarchy should be represented as tree structure.

When defining (1) Part objects and (2) Whole objects that act as containers for Part objects, clients must treat them separately, which complicates client code.

What solution does the Composite design pattern describe?

Define a unified Component interface for both part (Leaf) objects and whole (Composite) objects.

This enables clients to work through the Component interface to treat Leaf and Composite objects uniformly: Leaf objects perform a request directly, and Composite objects forward the request to their child components recursively downwards the tree structure. This makes client classes easier to implement, change, test, and reuse.

UML Diagram

The following diagram shows the Composite Pattern in UML notation. It is based on the corresponding chapter in the book “Head First Design Patterns“:

note top of Client The Client uses the Component interface to manipulate the objects in the composition. end note

note top of Component The Component defines an interface for all objects in the composition: both the composite and the leaf nodes. end note

note top of Component The Component may implement a default behavior for add(), remove(), getChild() and its operations. end note

note bottom of Leaf A Leaf has no children. end note

note left of Leaf Note that the Leaf also inherits methods like add(), remove() and getChild(), which do not necessarily make a lot of sense for a leaf node. We are going to come back to this issue. end note

note bottom of Leaf A Leaf defines the behavior for the elements in the composition. It does this by implementing the operations the Composite supports. end note

note bottom of Composite The Composite’s role is to define behavior of the components having children and to store child components. end note

note right of Composite The Composite also implements the Leaf- related operations. Note that some of these may not make sense on a Composite, so in that case an exception might be generated. end note @enduml

[/plantuml]

PlantUML Sources

PlantUML is a tool allowing users to create UML diagrams from a plain text language. Here are the PlantUML sources for the above software pattern: